This invention relates to magnetrons.
All vacuum tubes reach the end of their life when the electron emissive material on the cathode is entirely lost by evaporation or sputtering. All vacuum tubes except for magnetrons and other crossed beam tubes lose their active cathode material mainly by evaporation. Evaporation depends entirely on the temperature at which the cathode is operated and not on the running level so if the cathode temperature is known the life of the device can be predicted sufficiently accurately for most users. The life of magnetrons and similar tubes is difficult to predict, because the cathode material is lost mainly by sputtering.
Sputtering means that the cathode material is lost by ion bombardment. Ions are created in magnetrons, and in other vacuum tubes, by the collision of electrons with gas atoms, the gas atoms evaporating from the vacuum envelope and the material of the cathode. Negative ions will be attracted to the positive parts of the vacuum device, and positive ions will be accelerated towards the cathode and cause the loss of material. Magnetrons suffer much more from cathode damage by sputtering than do other vacuum devices, because the cathode is centrally located in the area where ions are likely to be created, current densities, implying more electrons to ionise atoms, are higher than in beam tubes, the cathode is large compared to that in an equivalent beam tube and is a source of gas for evaporation, and the vacuum volume is much less than in an equivalent beam tube, so residual gas densities will be higher.
The rate of sputtering is dependent on: the quality of the device processing, since the residual gas levels can be reduced by processing at higher temperatures and for longer; power rating (more cathode current implies more electrons which can cause ionisation); temperature of the vacuum envelope (since raising its temperature will increase the gas level); input and output conditions; and fault events (such as arcing which can cause local overheating, thus producing gas).
Magnetrons are often used at different power outputs on an hour by hour basis (for example when used in medical linear accelerators different treatment types require significantly different power levels, surface cancers needing low power and X-ray treatment needing high power) and this combined with variations in running temperature in use and any variation in processing makes the life variable and unpredictable.
Magnetrons are used as the microwave source in radar, medical linear accelerators and some industrial processes. In all these the user needs to carry out preventive maintenance to reduce unplanned downtime. To minimize the risk of equipment failure the user would like to replace items with a limited life very shortly before they fail. To do this the life needs to be predictable regardless of different operational conditions and tube history.